Advancement in emission technology for internal combustion engines has resulted in significant reductions in total vehicle emissions. In general, automotive emissions applications employ an exhaust mounted catalytic converter for reducing regulated exhaust constituents such as Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NO.sub.2). Ideally, the catalyst alters the rate of the converting, chemical reaction without itself being consumed or changed by that reaction. In reality, catalysts applied in automotive emission controls may change in use, and may become inactive or seriously degraded, requiring replacement. Since catalyst degradation is not predictable solely on mileage accumulation or determinable by external observation, a means by which the operation of the converter may be assessed, without undue intrusion or disassembly of the vehicle exhaust system, has been sought.
In one converter test method, tailpipe exhaust emissions levels are compared to engine-out exhaust emissions levels. Reductions in measured emissions between engine-out and tailpipe are attributed to catalyst activity. This method is intrusive in that it requires access to the exhaust flow upstream of the converter requiring a tap or a hole be introduced into the exhaust system establishing the potential for leakage.
A second method indirectly assesses catalytic activity by comparing the temperature of the exhaust gas entering the converter to that of the exiting exhaust gas. Catalyst activity within the converter should generate additional heat (approximately 200.degree. F.) during the conversion process which would be absent in the presence of a degraded catalyst. This method requires catalyst hydrocarbon loading sufficient to produce a temperature differential which is measurable and, as a result, has the potential for damaging the catalyst.
Another method indirectly assesses converter activity by measuring the efficiency of the chemical reaction taking place within the converter, which determines residual oxygen and the quantity of carbon monoxide produced in the unit. The method uses external control to vary the engine air/fuel mixture to obtain maximum exhaust CO.sub.2. Such external control may involve propane enrichment to establish stoichiometric operation in the converter during engine operation which is required for proper catalyst efficiency. Such a test requires that the engine and exhaust system be operating properly prior to testing. Fueling or ignition errors by the engine or air leaks in the exhaust system will give inaccurate readings resulting in replacement of a properly functioning catalytic converter.
The current tests for field determination of catalyst activity each suffer from the disclosed disadvantages. A primary disadvantage is the inability to isolate testing of the converter from the operation of the engine to thereby avoid failures of properly functioning converters caused by an improperly functioning engine or exhaust system.